Recent progress in sensor technology has allowed low Earth orbit (LEO) satellites to shrink significantly in size, disrupting a legacy industry where traditional satellites cost 500 million dollars to 1 billion dollars to build and launch. Major investments are being made to address the new opportunities that this provides for data collection, and many companies are launching nanosatellites and/or microsatellites into LEO to capture this opportunity. The rapidly expanding satellite infrastructure is generating vast amounts of data, reaching nearly 20 PB/year in 2014, with no signs that the trend will level off. To bring all that data down from LEO requires an average communication rate of 5 Gb/s, continuously, and that demand will continue to grow.
Typically, most satellites download data via space-to-ground radio-frequency (RF) links, communicating directly with fixed ground stations as the satellites fly within range. The current ground station infrastructure has several key limitations that present significant challenges as the satellite industry continues to grow. Satellite-to-ground communications are “line-of-sight,” meaning that ground stations must receive data only from satellites that are directly above the local horizon. The duration of a satellite passes over a ground station depends on the altitude of the satellite and the distance between the ground station and the ground track of the satellite. With satellites in LEO, the maximum pass duration is typically less than ten minutes.
The frequency of passes is strongly dependent on the satellite orbit parameters and the location of the ground station. For example, a satellite in equatorial orbit will pass over an equatorial ground station on each orbit. This means that with a typical orbital period of 90 minutes, the satellite will pass the ground station 16 times per day. Similarly, a satellite in a polar orbit will pass over a ground station located at the North Pole once per orbit. On the other hand, the satellite in polar orbit will pass over the equatorial ground station between two and four times per day depending on the alignment of the ground track with the location of the ground station.
However, it should be noted that the satellite in equatorial orbit will never pass over the polar ground station. Most LEO satellites are in orbits at some inclination between equatorial and polar, and most ground stations are located at latitudes well south of the North Pole. As such, the pass frequency for any given satellite over any given ground location will typically be three to five times per day for ground stations that are not at high latitude (above about 60 degrees) and not at latitudes higher than the orbital inclination of the satellite.
The consequence of limitations on pass duration and frequency is that a satellite will be within communication range of a given ground station for no more than 10 percent of a day, and typically for less than 2 percent of the day. These constraints on pass duration and pass frequency are driven by orbital dynamics and can be overcome only by increasing the number of ground stations or locating the ground stations at very high latitudes. However, increasing the number of ground stations requires a large amount of capital investment. Furthermore, avoiding downlink constraints requires a large number of geographically diverse ground stations that are inherently underutilized.
To compensate for the limitations on ground contact time, the data transmission rate during what contact time is available is increased. High data rates in the RF require some combination of high transmitter power and high-gain antennas on the satellite and the ground station. High power transmitters and high-gain antennas on the space segment are constrained by power and mass limitations on the satellite. High-gain antennas on the ground are not mass limited, but tend to be very large (10 meters or more in diameter) and require significant capital investment.
As data produced in LEO increases substantially with more satellites launched, downlink infrastructure must grow to meet demand. However, a more fundamental limitation to downlink rates will be encountered in the future, simply due to the overuse of available RF bandwidth in the space environment. Furthermore, simply adding new RF ground terminals will not solve the problem, because the ground stations will start to interfere with one another. Similarly, RF bandwidth is constrained on the space side. For example, when two satellites are relatively close to one another, their RF signals can interfere.
For new satellite companies leveraging advances in satellite costs, capital investment for an extended ground station network is particularly burdensome because the size and cost of the ground network does not scale with the size of the satellites. Ground station costs have not scaled at the same rate as satellite costs, requiring significant investment to match growth in satellite capacity.
Laser communication has the potential to provide data rates adequate to handle all the data generated on orbit for the foreseeable future. However, current laser communication technology requires installation of expensive laser transmitters on each satellite, and places operational constraints on the satellite (pointing, jitter, etc.) that are often beyond the capability of budget satellites.
Thus, a laser communication system that can support a broad range of satellites at a moderate cost and without placing an undue burden on the satellites may be beneficial.